Electrophoretic deposition of ceramic coatings

ABSTRACT

1. A PROCESS FOR ELECTROPHORETICALLY DEPOSITING A CERAMIC COATING ON AN IRON-CONTAINING WORKPIECE COMPRISING CONTACTING SAID WORKPIECE WITH AN AQUEOUS SOLUTION OF A WATER-SOLUBLE SALT OF A METAL BELOW IRON IN THE ELECTROMOTIVE SERIES TO DEPOSIT CHEMICALLY LAYER OF SAID METAL ON THE IRON-CONTAINING WORKPIECE, ELECTROPHORETICALLY DEPOSITING A CERAMIC COATING ON SAID METAL LAYER FROM A DISPERSION OF CERAMIC PARTICLES, AND MAINTANING AN ALKALINE PH IN SAID DISPERSION OF AT LEAST 11 TO PROVIDE GREATER THROWING POWER WITHOUT DISCOLORING THE RESULTING CERAMIC COATING.

United States Patent 3,841,986 ELECTROPHORETIC DEPOSITION OF CERAMIC COATINGS Richard G. Rion, Parma Heights, and Louis J. Gazo, Jr., Independence, Ohio, assignors to Ferro Corporation,

Cleveland, Ohio No Drawing. Filed Nov. 16, 1972, Ser. No. 307,140 Int. Cl. B01k 5/00 US. Cl. 204-181 9 Claims ABSTRACT OF THE DISCLOSURE An improved process is disclosed for electrophoretically depositing a ceramic coating on an iron-containing workpiece, characterized by chemically depositing on the workpiece prior to the electrophoretic deposition a layer of a metal below iron in the electromotive series. The workpiece is merely exposed to an aqueous solution of a water-soluble, ionizable salt of the metal. Optionally, the electrophoretic slip may contain colloidal silica, and the workpiece may be contacted after the electrophoretic deposition with an aqueous solution of a water-soluble salt of a strong inorganic acid to remove trace amounts of iron or iron compounds.

BACKGROUND OF THE INVENTION Electrophoretic deposition is a known phenonemon in which an electric potential is established between two electrodes immersed in a liquid dispersion of charged particles. Under the influence of a direct current electric potential, the particles migrate toward one of the electrodes where their charge is neutralized and the particles deposited as an adherent coating on the electrode. Electrophoretic deposition is commonly used for coating a ceramic material such as frit on metal, as in producing cermets. Electrophoretic deposition is generally regarded as superior to conventional, dipping, brushing or spraying techniques, since the deposition is rapid and more effectively coats all surfaces of a workpiece. Electrophoretic deposition is also a convenient way to coat completely the inside area of small holes in a workpiece Where failure of coatings by other techniques normally first take place.

Because ceramic or enamel coated metal products are often used in kitchens, bathrooms, and in other areas where appearance is a consideration, a white ceramic coating is much preferred because of its esthetic customer appeal. For the same reasons, an off-white or dirtywhite coloration is undesirable and has negative customer appeal.

During electrophoretic deposition on an iron-containing workpiece, iron usually as iron oxide diffuses from the surface of the metallic workpiece or ware being coated into the ceramic coating and becomes a part of it. Unfortunately, the presence of iron or the more reddish iron oxide materially affects the color of the ceramic coating and, especially in the case of white ceramic coatings, adversely affects the color desired as well as the surface of the fired coating.

Additionally, some techniques in electrophoretic deposition increase the undesired presence of iron in the ceramic coating. For example, one practice comprises predipping the workpiece in an acid activator solution, such as a sulfuric acid solution, to remove rust, for example, before applying the ceramic coating by electrophoresis. This appears to increase the tendency of iron to migrate from the workpiece into the ceramic coating material and into the electrophoretic dispersion, resulting in dis coloration of the finished, fired ceramic coating.

A related problem concerns the throwing power of an electrophoretic dispersion while maintaining good enamel ice color and surface qualities. Throwing power may be considered to be the power of the dispersion to deposit a coating of substantially the same thickness on different areas of the workpiece, even though such areas lie at different distances from the cathode. Throwing power is affected by the alkalinity of the electrophoretic medium, the greater the alkaline pH value the greater the throwing power. However, in using activators which increase the tendency of iron and iron oxide todiffuse into the ceramic coating, the higher pH values necessary for good throwing power cannot be used, since fired finished coatings of poor quality surface then result. The practice therefore has been to operate at a lower alkaline pH than might otherwise be the case to reduce discoloration of the fired ceramic coating, even though this results in a sacrifice of throwing power.

SUMMARY OF THE INVENTION A principal object of the invention, therefore, is to provide an improved process for electrophoretic deposition of ceramic coatings which results in coatings of improved quality as well as an improved color and superior gloss, and which further enables the use of electrophoretic dispersions of relatively high alkaline pH to provide excellent throwing power without deleterious results.

In the present process, a pre-dip of the workpiece in an acid solution prior to electrophoretic deposition is entirely eliminated. In lieu of the acid treatment, a layer of a metal below iron in the electromotive series, such as copper, is chemically deposited onto the workpiece from a water-soluble, ionizable compound of the metal. Thereafter, ceramic particles such as frit are electrophoretically deposited as before. The process can be carried out in alkaline pH electrophoretic dispersions having a pH as high as about 12. 1

Other procedural steps have been found to contribute as well to improved quality and an enhanced white color in a ceramic coating electrophoretically deposited in accordance with the present invention. For example, the electrophoretic dispersion may contain colloidal silica in addition to the ceramic particles. Also when, in spite of efforts to prevent its presence, trace amounts of iron or iron oxide reach the ceramic coating, contacting the ceramic bisque after the electrophoretic deposition with an aqueous solution of a water-soluble salt of a strong inorganic acid, such as monoammonium phosphate, has been found to remove such trace amounts Without attacking the ceramic or enamel bisque and resulting in improved color in the fired coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In one form, the present process comprises the following steps: activating the surface of a workpiece by chemically depositing thereon a layer of a metal below iron in the electromotive series, rinsing the metal coated surface, electrophoretically depositing a ceramic coating onto such surface from an electrophoretic dispersion containing colloidal silica, post-dipping the electrophoretically treated workpiece in an aqueous solution of a strong inorganic acid and preferably a water-soluble salt of such acid, and firing the workpiece to provide the ceramic coating. Of these steps, at least those of using colloidal silica in the electrophoretic dispersion, and of treating the electrophoretic-deposited surface with a water-soluble salt are optional.

Apparatus for carrying out the invention consists essentially of a suitable tank, or in the case of a continuous line, of a suitable elongated linear tank or trough for holding the slip, a source of direct current, the interior of the tank generally being connected across the electrical source to render it cathodic, and suspending means for 3 carrying the workpiece to be coated, the latter being made the anode.

Considering the steps of the process in further detail, an acid-activation is entirely eliminated. Instead, a workpiece such as an article of cast iron or mild steel is dipped into an aqueous solution of a water-soluble, ionizable compound of any metal below iron in the electromotive or displacement series. During immersion of the workpiece in the solution, a relatively thin layer of metal is chemically plated onto the workpiece. In this manner, a coating of one or more of such metals as cadmium, cobalt, nickel, zirconim, tin, lead, copper, mercury, or the like is readily obtained at virtually no operating cost in that only dipping of the workpiece is required in the absence of any externally applied force or energy. Elaborate or especially designed equipment is completely avoided and the time involved is only a matter of seconds.

Any ionizable, water-soluble compound of the indicated metals can be used, since it is necessary only to furnish metal ions that may act in accordance with the electromotive series. Normally, metal salts of inorganic acids, where water-soluble, are well suited for this purpose, such as the metal sulfates, chlorides, nitrates, phosphates, carbonates, and the like. The acetates can also be used where water-soluble. The substitution of iron for the metal of the metal salt in accordance with electromotive series is normally facilitated if the aqueous solution is acidic. When the salt is one of a strong inorganic acid, this may be sufiicient to make the solution acidic.

The concentration of the metal salt in the aqueous solution and the time of exposure of the solution to the workpiece are not at all critical and need be sufficient only to deposit a preferably continuous film of the metal onto the workpiece. For example, the metal film may be about 0.05 mil to 0.5 mil in thickness. Thicker films are only wasteful of the metal. Aqueous solutions containing from about 0.1% to about 5% by weight of the metal salt and immersions of the workpiece in the aqueous solution for about 3 to about seconds may be used.

Following deposition of a metal layer on a surface of the workpiece, the surface is rinsed. The rinse may be either by water or, to insure neutrallization of traces of the acidic aqueous solution which may be left, by a mild alkaline aqueous solution, or successively by the alkaline solution and then by water. An alkali metal carbonate, such as sodium carbonate, is preferred for preparing the mild alkaline aqueous solution if one is employed.

The workpiece is now ready for electrophoretic deposition. Any conventional frit for ceramic or enamel coating may be used in practicing the invention, although the invention is especially adapted for use with white frit. As an example, the frit particles may be on the order of less than 100 microns in dimension, and preferably, less than 40 microns. While organic materials can be included in the dispersion, the process works entirely satisfactorily in a strictly aqueous medium.

The frit may be milled to an aqueous slip in a conventional ball mill. A range of typical mill additions, for example, may comprise the following as shown by Table A.

TABLE A Mill additions Weight percent *Silica, clay, etc.

Resultant slip from above: Specific gravity-l.5-l.9. Fineness rangel gram retained on 200 mesh screen per 50 cc. slip to 1 gram retained on 400 mesh screen per 50 cc. slip. Typical fineness is 3 grams retained on 400 mesh screen per 50 cc. slip.

As is well known in enamel technology, there is a wide range of other, acceptable electrolytes, and mill agents which may be tolerated in the mill addition.

To the base milling of Table A the following additions may be made, if needed, for the purpose indicated: sufficient water to adjust the specific gravity from about 1.25 to about 1.45; sodium hydroxide to adjust the pH from about 8.0 to about 12.0; sodium chloride or sodium aluminate to adjust the electrical resistivity of the medium from about 150 to about 500 ohms per cubic centimeter. These values are not critical to practicing the present invention.

It has been found that the presence of colloidal silica in the electrophoretic dispersion further improves the color and quality of the finally fired ceramic coating. Colloidal silica may be added to the range of materials of Table A in amounts up to about 10% by weight of the materials added to the aqueous medium to produce the slip. An amount of about 5% is preferred.

For electrodeposition, direct electrical current is used. The suspension bath closes the circuit between anode and cathode. The article being coated is made the anode of the electrical circuit while the cathode may be the interior surface of the tank, or separate sheets or plates of a conducting material such as stainless steel, mild steel, or copper. The voltage between the article being coated and the cathode is adjusted to a value such that the current density on the article being coated is in the range of 5 to 100 amperes per square foot, preferably between 15 to 60 amperes per square foot.

The voltage required is generally in the range of 10 to 200 volts, with the exact voltage requirement depending upon the size of the cathode, the size of the article being coated, the distance between the cathode and the article being coated, and the electrical resistivity of the suspension bath. For the suspension bath described above the electrical resistivity will be approximately 300 ohms centimeters. Voltage required also depends on the separation of the article being enameled from the submerged electrode. For one inch separation the range of voltage is 2-50 volts. For eighteen inches of separation the range of voltage is 50-300 volts.

When the electrodeposition container is an electrically insulated taken 6 inches wide, 8 inches long, and 18 inches deep, and the article being coated is a 4 inch square panel of 20 gauge enameling iron, the voltage required for a current density of 20 amperes per square foot is approximately 25 volts. Obviously, the voltage may be varied to provide the ideal current density in slips of varying resistivity and to accommodate the other variables influencing electrodeposition.

The thickness of the deposited coating may be controlled by regulating the quantity of electricity per unit area passing through the suspension bath. Since the quantity of electricity per unit area is the product of the current density and the time during which the current flows, the thickness of deposited coating is controlled by both the current density and the deposition time. Times of deposition at a current density of 20 amperes per square foot have been found to vary from 5 to 50 seconds for a coating whose thickness after firing it three mils. A thicker, or thinner, coating requires a proportional increase, or decrease in deposition time.

In accordance with the present invention, the electrophoretic dispersion may have a relatively strong alkaline pH, which enables greater throwing power to be obtained, without discoloring the finally fired ceramic coating. For example, the-alkaline pH of the electrophoretic dispersions may be as high as about 12. Such relatively high alkaline pH may be obtained by adding sodium hydroxide, potassium, hydroxide, and the like, to the electrophoretic dispersion.

The length of time during which the workpiece is in the dispersion with a direct current potential applied is in the range of about 5 to about seconds. The time requirement is dependent upon the size and shape of the workpiece and upon the thickness of the coating to be deposited. After depositing a coating, the workpiece is removed from the dispersion and rinsed with water to remove any loose particles of glass frit and to prevent edge beading. Following a rinse, the coating is dried as by being placed in an oven equipped with infrared lamps. Still other conventional drying means may be used.

Another advantage of the present invention is that relatively strong wet bisques are obtained which enable the bisque to be handled with somewhat less care than might otherwise be the case. Also, in spite of efforts to the contrary, some traces of iron or iron oxide may appear in the electrophoretically deposited coating. The strength of the present wet bisque is such that the coated workpiece may be treated to remove the trace iron. This has been found to be best accomplished by contacting a bisque with a strong inorganic acid, such as sulfuric, nitric, or phosphoric acids, and removing the iron as the iron salt of the acid, such as iron phosphate, without appreciable damage to the bisque.

Aqueous solutions of these acids up to about 4% by weight are usually not too strong, but substantially stronger acid solutions may damage the bisque. For this reason water-soluble salts of the acids are preferred, for example, water-soluble salts which generate in aqueous solution a pH of about 3 to about or are used in amounts to generate a pH within that range. A preferred salt is mono-ammonium phosphate. The strength of the aqueous solution is not critical and may range from about 0.1% to about 10% by weight of the salt. Time of exposure need be no longer than sufiicient to remove the trace of iron and normally ranges up to about 10 to 60 seconds dependent upon strength of solution used- Final firing of the electrophoretically deposited workpiece may be carried out by convention methods and for conventional times and temperatures. As an example, firing of deposited coating may be from 2 to 5 minutes at 1400 F. to 1600 F.

The following example is intended to illustrate the in vention and should not be construed to impose limitations on the claims.

WORKING EXAMPLE A square, clean plate, four inches on a side, of zero carbon steel was immersed in the following activator solution:

Weight] Weight gallon (02.) percent Hydrated copper sulfate 1 0. 74 Sulfuric acid, 6 Baume 0. 5 0.37 Water Balance Balance A thin layer of copper metal chemically plated onto the metallic plate. The plate could be immersed for about to about 30 seconds, depending on the thickness of the layer desired. As a rule a thickness of about 0.5 mil sufiices for most purposes. After immersion, the plate was rinsed with a neutralizing solution of 0.5 ounce of sodium carbonate in a gallon of water to remove traces of acid.

A white frit was prepared by conventional smelting and fritting procedures and had the analysis shown by Table B.

6 This frit was then milled to an aqueous slip in a conventional ball mill, using the mill addition shown by Table C.

TABLE C Parts by weight Frit (Table B) 100 Silica 5 Bentonite 0.5 Gum tragacanth 0.5 Water 45 The milling was carried out to a particle size within the fineness range of Table A. The resulting slip was added to a conventional deposition tank along with these other additions: sufiicient water to provide a specific gravity of about 1.35; sufiicient sodium hydroxide to provide a pH of about 11.0 to about 11.5; and sufficient sodium chloride to provide a resistivity of about 200 to about 250 ohms per cubic centimeter. The resulting electrophoretic bath was now ready for enamel applications.

The previously copper coated workpiece was then immersed in the bath as an anode and direct current energy applied within the following example electrical parameters:

Range Voltage 25 to volts. Current Density 20 to amperes per square foot. Time 10 to 40 seconds.

The workpiece was next removed from the bath and sprayed with water to remove any loose particles of frit and then immersed in a six percent by weight aqueous solution of mono-ammonium phosphate for about 30 seconds to insure elimination of any yellow iron color in the resulting ceramic bisque. The workpiece was then placed for 15 to 2-0 minutes inan oven at F. to 300 F. to dry and finally fired at 1450 F. for 3.5 minutes.

Products coated with ceramics in accordance with the present process have appreciably improved color and gloss even though relatively high alkaline pH electrophoretic dispersions or baths are used resulting in better throwing power. Operating a bath at relatively high pH values essentially increases the length of time needed to produce an enamel coat of predetermined thickness, even though improved throwing power is obtained. When using prior activators for a workpiece, such as a sulfuric acid activator, followed by use of deposition baths of relatively high pH values, the added time caused a pullthrough of iron from the workpiece causing severe discoloration and poor surface quality in the fired enamel coating.

The white enamel coatings of the present invention are cleaner in appearance and have superior gloss as compared with white coatings obtained from either standard electrophoretic processes or electrostatic spraying, dipping, flow coating, and still other means of application. The fired appearance of the final product of the present invention is smooth and uniform and remarkably free of ripples or curtaining.

Although the forgeoing describes several embodiments of the present invention, it is understood that the invention may be practiced in other forms within the scope of the following claims.

We claim:

1. A process for electrophoretically depositing a ceramic coating on an iron-containing workpiece, comprising contacting said workpiece with an aqueous solution of a water-soluble salt of a metal below iron in the electromotive series to deposit chemically a layer of said metal on the iron-containing workpiece, electrophoretically depositing a ceramic coating on said metal layer from a dispersion of ceramic particles, and maintaining an alkaline pH in said dispersion of at least 11 to provide greater throwing power without discoloring the resulting ceramic coating.

2. The process of claim 1 in which said metal forming the metal layer is copper.

3. The process of claim 1 in which said water-soluble salt is copper sulfate.

4. The process of claim 1 in which said electrophoretic deposition is from an aqueous slip containing colloidal silica.

5. The process of claim 1 including contacting said workpiece after the electrophoretic deposition with an aqueous solution of a strong inorganic acid or a watersoluhle salt thereof.

6. The process of claim 5 in whichsaid water-soluble salt is a salt of phosphoric acid.

7. The process of claim 1 including firing the workpiece suhsequent to said eletcrophoretic deposition to form a ceramic coating.

8. The process of claim 1 in which said electrophoretic deposition is from an aqueous slip containing colloidal silica, and including contacting said workpiece after the electrophoretic deposition with an aqueous solution of a strong inorganic acid or a water-soluble salt thereof.

9. The process of claim 1 in which said ceramic coating is white.

References Cited UNITED STATES PATENTS T. M. TUFARIELLO, Primary Examiner 

1. A PROCESS FOR ELECTROPHORETICALLY DEPOSITING A CERAMIC COATING ON AN IRON-CONTAINING WORKPIECE COMPRISING CONTACTING SAID WORKPIECE WITH AN AQUEOUS SOLUTION OF A WATER-SOLUBLE SALT OF A METAL BELOW IRON IN THE ELECTROMOTIVE SERIES TO DEPOSIT CHEMICALLY LAYER OF SAID METAL ON THE IRON-CONTAINING WORKPIECE, ELECTROPHORETICALLY DEPOSITING A CERAMIC COATING ON SAID METAL LAYER FROM A DISPERSION OF CERAMIC PARTICLES, AND MAINTANING AN ALKALINE PH IN SAID DISPERSION OF AT LEAST 11 TO PROVIDE GREATER THROWING POWER WITHOUT DISCOLORING THE RESULTING CERAMIC COATING. 